This currently means macOS 10.11 only, which is only used on Travis CI
and Python 2 installation there is broken anyhow, so this doesn't make
anything worse than it already is.
Python package hosting seems to randomly decide which version of the
package to return to pip running under Trusty: in addition to werkzeug
(see last commit), the version of flask has spontaneously changed from
1.0 to 1.2 in the latest build too.
Just hardcode the versions of all dependencies in the last successful
build and hope they keep working for longer than a day.
Crazily, pip installs seem to be non-deterministic as installing httpbin
in https://travis-ci.org/github/wxWidgets/wxWidgets/jobs/765946720
pulled in werkzeug 0.16.1 which works with Python 3.4, but doing it
again tries to use werkzeug 1.x which isn't.
Fix the version explicitly to work around this.
Trying to use Python 2 under Trusty is hopeless, as pythonhosted.org has
already started dropping support for non-SNI clients and will drop it
completely soon, see https://status.python.org/incidents/hzmjhqsdjqgb
and https://github.com/pypa/pypi-support/issues/978, and SNI is not
available in the system Python 2 version (2.7.6, which is less than
2.7.9 in which it was added).
Note that we still can't use Python 3 everywhere as long as we have
macOS 11 builds as it's not available there.
Amazingly, updating a package has no problems with installing a newer
version incompatible with the current Python version, such as installing
pip 21 when using Python 3.5.
